1. Field of the Invention
The present invention relates to a catalyst for purifying exhaust gases and a process for purifying exhaust gases. More particularly, it relates to the catalyst and the process which can efficiently purify nitrogen oxides (NO.sub.x) in the exhaust gases whose oxygen concentrations are at the stoichiometric point or more than required for oxidizing carbon monoxide (CO) and hydrocarbons (HC) therein.
2. Description of Related Art
As catalysts for purifying automotive exhaust gases, there have been employed 3-way catalysts so far which oxidize CO and HC and reduce NO.sub.x to purify the exhaust gases. For example, the 3-way catalysts have been known widely which comprise a heat resistant support formed of cordierite, a catalyst carrier layer formed of gamma-alumina and disposed on the support, and a noble metal selected from the group consisting of Pt, Pd and Rh and loaded on the catalyst carrier layer.
The purifying performance of the 3-way catalysts for purifying exhaust gases depends greatly on the air-fuel ratio (A/F) of automotive engine. For instance, when the air-fuel weight ratio is larger than 14.6, i.e., when the fuel concentration is low (or on the fuel-lean side), the oxygen concentration is high in exhaust gases. Accordingly, the oxidation reactions purifying CO and HC are active, but the reduction reactions purifying NO.sub.x are inactive. On the other hand, when the air-fuel ratio is smaller than 14.6, i.e., when the fuel concentration is higher (or on the fuel-rich side), the oxygen concentration is low in exhaust gases. Accordingly, the oxidation reactions are inactive, but the reduction reactions are active.
Moreover, when driving automobiles, especially when driving automobiles in urban areas, the automobiles are accelerated and decelerated frequently. Consequently, the air-fuel ratio varies frequently in the range of from the values adjacent to the stoichiometric point (or the theoretical air-fuel ratio: 14.6) to the fuel-rich side. In order to satisfy the low fuel consumption requirement during the driving conditions such as in the above-described urban areas, it is necessary to operate the automobiles on the fuel-lean side where the air-fuel mixture containing oxygen as excessive as possible is supplied to the engines. Hence, it has been desired to develop a catalyst which is capable of adequately purifying NO.sub.x even on the fuel-lean side (i.e., in the oxygen-rich atmospheres).
In view of the aforementioned circumstances, the applicants of the present invention applied for a Japanese Patent for a novel catalyst under Japanese Patent Application No. 4-130,904 (Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652). On this catalyst, there are loaded an alkaline-earth metal oxide and Pt. In the catalyst, during the fuel-lean side (i.e., in the oxygen-rich atmospheres) driving, NO.sub.x is adsorbed on the alkaline-earth metal elements, and it reacts with the reducing gas such as HC and the like to be purified. As a result, the catalyst exhibits superb NO.sub.x purifying performance during the fuel-lean side (i.e., in the oxygen-rich atmospheres) driving.
The catalyst proposed in Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652 is believed to provide the advantageous effect as follows: the alkaline-earth metal oxides, for example, barium oxide loaded on the support, react with NO.sub.x to produce nitrates, e.g., Ba(NO).sub.3. Thus, NO.sub.x is adsorbed on the support of the catalyst in the form of the alkaline-earth metal nitrates.
However, the exhaust gases usually contain SO.sub.2 which is produced by burning sulfur (S) contained in the fuel. Further, the catalyst ingredient oxidizes SO.sub.2 to SO.sub.3 in the oxygen-rich atmospheres (i.e., on the fuel-lean side). Still further, SO.sub.3 reacts readily with water vapor also contained in the exhaust gases to produce sulfuric acid. It has been revealed that the resulting sulfuric acid reacts with the alkaline-earth metal elements to produce alkaline-earth metal sulfites and alkaline-earth metal sulfates, thereby poisoning the alkaline-earth metal elements. Specifically, when the alkaline-earth metal elements are turned into the sulfites and sulfates, they hardly adsorb NO.sub.x thereon. As a result, the catalyst proposed in Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652 suffers from a drawback in that it is deteriorated in terms of the NO.sub.x purifying performance after it is subjected to a durability test.